I found Truggy pipe for .28 engines, I have no idea how they work, maybe Neal has tested one...(?)
The price is right;
http://www.worldchampionproducts.com...pma280002.html
Do you know what rpm you need the engine to turn?
The onroad manifold I got measure about 45mm, it's from Hipex.
I'm not sure on how the pipe is mounted, but remember that Picco has an angled outlet. The "normal" offroad manifold is easy to tweak, but I doubt it's possible with an onroad manifold.